This invention is directed to a molded article, more particularly to a cam of a sintered powder metallurgically produced alloy for a camshaft for internal combustion engines, which is assembled according to the modular principle, as well as to a method for its production.
Throughout the specification, the numbers inside parentheses refer to publications numbered according to a list which is appended to this application.
The cams of camshafts of internal combustion engines are exposed to very heavy wear. To fulfill their task of controlling the engine, the wear during the whole of their service life should not exceed more than a few microns. In this connection, they must also withstand load cycles while insufficiently lubricated. The conventional method in the literature and in industry is the use of alloys with a high carbide content, which are produced either by powder metallurgical means from appropriate materials or by rapidly quenching cast iron. By these means, the abrasive, as well as the adhesive wear can be kept within limits.
Aside from mechanical stresses, cams are also subjected to thermal stresses. For this reason, the nature of the cams must be such that they maintain their hardness even after prolonged annealing. This can be achieved by hardening and subsequently annealing at a temperature above the operating temperature. Even under operating conditions at which deficient lubrication occurs and which promote adhesive wear, the cams must exhibit excellent operating behavior.
For some years now, particularly since camshafts assembled according to the modular system have come into vogue for internal combustion engines (1, 2), the debate about the wear in the cam counterbody system has intensified. Aside from references to the fact that wear in this system is very sensitive to and depends on lubrication (3) and the finishing by grinding or superfinishing (4, 5), there is a large number of publications, which attempt to solve the problem on the basis of material development.
For a promising start, it is first of all necessary to analyze the wear problems of this system. It has been pointed out in numerous publications (3, 6, 7) that wear makes its appearance above all as polishing wear, pitting and scoring.
Polishing wear is one form in which abrasive wear appears. By using appropriately fine abrasives, a very small amount is removed and the grooves formed are very small. The cam, so worn, appears to be brightly polished, the roughness of the worn regions generally being significantly less than that of the undamaged (ground) regions. The polishing wear can be caused as 3-body wear by quartz dust in the oil. Sand is one of the most frequently occurring abrasive materials in technology. Since polishing wear also occurs under experimental conditions, for which contamination of the oil can be excluded, there must also be yet another mechanism. Polishing wear can obviously also be aided by a rough counter-body, which contains no carbide.
Pitting is a consequence of surface fatigue. The dynamic pressure load on the cam surface, which is the result of the kinematics, can lead to local fissure spreading. These fissures extend below the surface and run together with other fissures or emerge again from the surface. A consequence is the formation of relatively large wear particles and small pits on the surface. This wear phenomenon can be furthered by additives in the oil (3), if the additives facilitate the spread of the fissures, for example, by decreasing the surface energy.
Scoring is a consequence of adhesive wear, that is, the mutual welding of surfaces. It is favored by the use of martensitic parent substances and counter-objects (8) and through the use of plain oil. Experiments with increased springiness of the valve spring also favor scoring. Of 43 pairings, 26 failed due to scoring when plain oil was used. On the other hand, not a single pairing failed due to scoring when doped oil was used (8). As against this, failure due to pitting increased from 17 pairings to 35 pairings for doped oil (8).
Despite the frequent occurrence of pitting, less attention is paid to this wear phenomenon in investigations than to the other two. In principle, pitting itself does not affect the function of the cam (6). However, it decreases the bearing surfaces, so that the surface pressure increases, as a result of which failure due to scoring can be caused. Moreover, the pitting tendency can readily be recognized in short term tests with an increased load (7), while the results of polishing and scoring wear can be extrapolated only with extreme care (8, 9). Pitting therefore is not critical, as long as it occurs only to a slight extent. Moreover, it can be simulated easily in experiments.
Most publications are concerned with avoiding scoring wear and polishing wear. Moreover, all experiments aim at producing materials with a high proportion of carbide (2, 6, 8, 9, 10, 11, 12, 13). Due to their high hardness, carbides decrease the depth of penetration of the counter-body. By these means, the size of the wear particles and, with that, the possible rate of wear is reduced (14). The second effect is due to the low tendency to adhere, which is exhibited by the carbides. If the carbides constitute a sufficiently large proportion by volume, adhesion wear is avoided completely. Attempts to reduce cam wear by a solid lubricant, which is embedded in the cam, are not known.
The embedding of lubricants in sintered alloys has been used for a long time, in order to produce a self-lubricating bearing (15). For example, lead, which was introduced by impregnating it in a relatively complex alloy (Fe-Co-Mo-Ni-Cr-Si-C), is used. This alloy has proven its value when used in valve seats in internal combustion engines (16).
There has already been much discussion in the literature of copper as an alloying element, because it is an easily processed element (its oxygen potential is significantly less than that of iron). The mechanical properties (17, 18) or the dimensional behavior (19, 20), as well as homogenization (21) are discussed frequently. In conventional steel technology, copper is known as a material harmful to steel, since it promotes the tendency to develop red-shortness (22). In powder metallurgical manufacture, however, this type of failure does not play a role, as long as the molded articles do not have to be converted by sinter forging.
The effect of copper on the wear of sintered iron is significantly less than the effect of the density, at least when copper is admixed in amounts of 0 to 2% (23). Samples of different density were investigated in the Amsler Tribometer (two cylinders rolls with a slippage of 10% relative to one another). The atmosphere (air, argon or oxygen) has a decisive effect on the amount of wear. Wear in an oxygen atmosphere is greater by a factor of 72 than wear in an atmosphere of air. Since the wear under argon lies between the two values, it is very likely that water vapor has an effect in the experiments. The sintering conditions, which take place at 1120.degree. C., lead to the assumption that the copper is dissolved completely in the matrix.
The effect of admixing copper in amounts of 0 to 4% in various phosphorus containing sintered steels was also investigated (24). In the more highly alloyed variants (4% Mo, 4% Ni or 4% MCM, a master alloy of molybdenum, chromium and manganese), an addition of copper causes a decrease in the wear in the pin-disk test. In the less highly alloyed variants, the addition of copper has a relatively unsystematic effect. The effect of the copper is based on hardening the matrix. Although the sintered density decreases with increasing copper content, the hardness increases continuously with the increasing copper content. Because of the increase in hardness, it can be assumed that the copper is dissolved completely in the matrix. The decrease in density is also an indication of this. Copper leads to a decrease in density during sintering, if it is dissolved in the matrix and if pores remain behind in those places in which the copper was originally present.
The combination of a binding phase of copper, manganese or nickel or combinations thereof with very hard super-speed steel particles has also already been investigated (25). The structure, so produced, is more ductile than pure super-speed steel and has proven its value for applications in which there is wear.
In many other investigations (26, 27, 28, 29), copper serves as a model material for fundamental investigations. The finding that the rate of wear of copper when sliding dry against iron is less than that of nickel by a factor of 5, seems remarkable (28). This result indicates the slight tendency of copper-iron pairings to adhere and the good emergency running properties of copper, which are associated with this.
Molybdenum is to be found in very many P/M steels. The reason for the frequent use of 0.5% molybdenum is surely strictly practical in nature. A basic iron powder containing 0.5% molybdenum is commercially available. The deliberate admixture occurs in only the most infrequent of cases. Fe-P-Cu-Mo alloys with copper contents of up to 4% and molybdenum contents of 2% and 4% were also investigated (17). All alloying components were mixed in as elements. After a 1-hour sintering process at 1200.degree. C., the samples with 2% of molybdenum and 4% of copper had an irregular 2-phase structure. This inhomogeneity becomes even clearer if the molybdenum content is increased to 4%. Carbon retards the diffusion of Cu in Fe, but does not prevent the complete dissolution.
Numerous attempts to control the wear in the cam/counter object system are known. Up to now, all of them have been based on producing a carbide rich structure.